This invention relates to a shaft seal and in particular to a visco-type seal having one or more breakdown bushings encompassing the shaft of a rotary machine.
In many high pressure rotary machines, such as turbines and compressors, a high pressure oil stream is created about the shaft at one or both end closures of the machine. The oil establishes a barrier about the shaft which functions to prevent the high pressure gases contained within the machine from escaping to atmosphere. Typical of such a sealing arrangement is that disclosed by Damratowski in U.S. Pat. No 3,695,621, which issued October 1972. This type of sealing assembly normally consists of a number of bushing rings which loosely encompass the shaft. A first gas side bushing is positioned adjacent to the process gases on the high pressure side of the closure. One or more "breakdown bushings" are radially aligned behind the gas side bushing with the alignment extending outwardly towards the low pressure side of the closure. A sealing fluid, typically oil, is introduced between the gas side seal and the shaft under a positive pressure that is slightly higher than the pressure of the working fluids contained within the machine. An oil-to-gas interface is thus established for preventing the process gases from moving outwardly along the shaft. A clearance is maintained between the inner surface of each breakdown bushing and the outer periphery of the shaft whereby the sealing oil moves from the high pressure side of the sealing assembly towards atmosphere. As the oil moves beneath the breakdown bushings, the high pressure in the oil stream is uniformly broken down until it finally is reduced to atmospheric pressure.
The pressure dropped by the sealing fluid during the breakdown process is normally transmitted to each of the free-floating breakdown bushings. A relatively high axial load is thus induced in each bushing which forces the bushing laterally into contact against the stator which houses the bushing. Under high load conditions, the bushing can bind or become locked against the stator to establish a hydrodynamic stiffness in the system. The bushings thus act as bearings in regard to the shaft and become cross coupled to the rotor journal bearings. When this occurs, the natural frequency of the rotor system is drastically changed which, of course, has a harmful effect on the operation of the machine.